Re: [Patch v4] mm: thp: remove the defer list related code since this will not happen

Wei Yang
2020-01-19T02:24:50Z

On Sat, Jan 18, 2020 at 03:36:06PM -0800, David Rientjes wrote:
>On Sat, 18 Jan 2020, Andrew Morton wrote:
>
>> On Sat, 18 Jan 2020 07:38:36 +0800 Wei Yang <richardw.yang@linux.intel.com> wrote:
>>
>> > If compound is true, this means it is a PMD mapped THP. Which implies
>> > the page is not linked to any defer list. So the first code chunk will
>> > not be executed.
>> >
>> > Also with this reason, it would not be proper to add this page to a
>> > defer list. So the second code chunk is not correct.
>> >
>> > Based on this, we should remove the defer list related code.
>> >
>> > Fixes: 87eaceb3faa5 ("mm: thp: make deferred split shrinker memcg aware")
>> >
>> > Signed-off-by: Wei Yang <richardw.yang@linux.intel.com>
>> > Suggested-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
>> > Cc: <stable@vger.kernel.org> [5.4+]
>>
>> This patch is identical to "mm: thp: grab the lock before manipulating
>> defer list", which is rather confusing. Please let people know when
>> this sort of thing is done.
>>
>> The earlier changelog mentioned a possible race condition. This
>> changelog does not. In fact this changelog fails to provide any
>> description of any userspace-visible runtime effects of the bug.
>> Please send along such a description for inclusion, as always.
>>
>
>The locking concern that Wei was originally looking at is no longer an
>issue because we determined that the code in question could simply be
>removed.
>
>I think the following can be added to the changelog:
>
>----->o-----
>
>When migrating memcg charges of thp memory, there are two possibilities:
>
> (1) The underlying compound page is mapped by a pmd and thus does is not
> on a deferred split queue (it's mapped), or
>
> (2) The compound page is not mapped by a pmd and is awaiting split on a
> deferred split queue.
>
>The current charge migration implementation does *not* migrate charges for
>thp memory on the deferred split queue, it only migrates charges for pages
>that are mapped by a pmd.
>
>Thus, to migrate charges, the underlying compound page cannot be on a
>deferred split queue; no list manipulation needs to be done in
>mem_cgroup_move_account().
>
>With the current code, the underlying compound page is moved to the
>deferred split queue of the memcg its memory is not charged to, so
>susbequent reclaim will consider these pages for the wrong memcg. Remove
>the deferred split queue handling in mem_cgroup_move_account() entirely.
>
>----->o-----
>
>Acked-by: David Rientjes <rientjes@google.com>

Hi David,

The changlog looks awesome to me. Thanks ~

Hi Andrew

I see you queue this in you tree, do I need to rewrite a patch with better
changelog?

--
Wei Yang
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